Color photography in its most widely used form employs color negative film that is imagewise exposed in a camera. The color negative film includes a transparent film support and three superimposed layer units, for separately recording exposure within the blue, green and red portions of the visible spectrum. Each layer unit contains radiation-sensitive silver halide grains for selectively capturing exposing radiation in one region of the spectrum and dye-forming coupler. During processing light-struck silver halide grains containing a latent image are reduced to silver using a color developing agent. The blue, green and red recording layer units contain silver halide grains that form a latent image in response to exposure to blue, green or red exposure, respectively, and interact with coupler to form an image dye that has its principal absorption in the blue, green or red, respectively, resulting in a subtractive primary yellow, magenta or cyan dye image. As a result the multicolor image formed by the film is a negative of the scene photographed.
By altering either the silver halide grains (e.g., employing direct-positive emulsions) or the processing procedure (e.g., reversal processing) it is possible to produce dye upon reduction of only the grains that do not contain a latent image. This produces a positive dye image. The first multicolored films forming subtractive primary imaging dyes were produced by reversal processing.
Most positive multicolor images for viewing are produced in photographic elements having a reflective support--e.g., color print elements. The layer units are basically similar to those in the color negative elements described above, though differing in specifics, such as grain selections. Upon exposure of the color print through an image-bearing color negative film, a positive multicolor image (a negative of the exposing negative image) is produced in the color print.
The problem that delayed using color negative film and color print elements in combination to produce viewable multicolor images was that while the couplers produce dyes that have their principal absorption in the intended spectral region, the image dyes also exhibit significant absorption in adjacent spectral regions. When a second exposure occurs through the color negative film image, this color error is carried forward and added to the same inherent color error in the print element. Without correction, the result is unsaturated and "muddy" images, not achieving the degree of color fidelity required to be pleasing to the viewer of the color print image.
A solution to this problem allowed popular acceptance of the color negative and color print imaging system approach, which is now the most widely used approach. The solution resides in masking each dye image portion that lies outside its spectral region of principal absorption. If, for example, a green recording layer unit contains a dye-forming coupler that produces a dye that absorbs principally in the green, but also exhibits significant unwanted absorption in the blue, the incorporation of a yellow (blue absorbing) masking coupler can be used to raise blue absorption to a relatively constant level, independent of exposure. In areas in which the green recording layer unit is not light exposed the blue absorption due to the presence of the masking coupler is maintained after processing. In areas in which the green recording layer unit is exposed, the masking coupler is converted to a magenta dye along with the dye-forming coupler. The loss of blue absorption due to consumption of the masking coupler is chosen to balance the increase in blue absorption attributable to the blue absorption of the primarily magenta dye formed by the dye-forming coupler. Hence, after processing there is little, if any, image discrimination (Dmax-Dmin) in the blue region of the spectrum attributable to the green recording layer unit. Combinations of masking couplers are used to mask imagewise distributions of image dye absorptions that are outside the spectral region of principal (intended) absorption. Hansen U.S. Pat. No. 2,449,966 provides an example of early disclosure of masking couplers and their role. L. Stroebel, J. Compton, I. Current and R. Zakia, Photographic Materials and Processes, Focal Press, Boston/London 1986, Section 16.8 Color-Negative Masking, pp. 525 to 528, also provides a discussion of masking couplers.
Once masking couplers have been introduced to eliminate image patterns of unwanted absorption, blue, green and red characteristic curves produced by step wedge exposure, processing, and blue, green and red transmission through the negative working multicolor film differ in minimum density and usually in gamma To achieve an acceptable positive color print image by exposure through the color negative film, it is conventional practice to construct the color print to compensate exactly for the offsets in the color balance of the color negative film created by the presence of masking couplers. Once the compensating offsets have been built into the color print elements, a standard offset has been established. As a result color negative films sometimes contain uniform distributions of preformed masking dye to adjust color balance offsets to the standard profile for which the color print material is constructed. Stroebel et al, cited above, Section 16.12 Negative Color Films, provides a further description of the color balance in conventional color negative film construction.
With the emergence of computer controlled data processing capabilities, interest has developed in extracting the information contained in an imagewise exposed color negative film instead of proceeding directly to a viewable image. For example, a pattern of use that has emerged among professional photo-graphers, particularly photojournalists, is to take large numbers of pictures on location using color negative film, process the film to produce color negatives, and then select what appear to be the most promising negatives for scanning--i.e., creation of a corresponding digital image that can be transmitted to a remote location and electronically manipulated and used in any number of ways, including forming a color print. The very large number of pictures taken as well as the time and expense limitations of scanning preclude converting more than a very small fraction of the total images obtained to digital form.
Not surprisingly, one of the significant problems that has emerged in digital conversion of color negative film images is judging from visual inspection which image will be most suitable when converted to a color positive for viewing. The photographer has not only to contend with the fact that the image being judged is a negative of the image intended for final viewing, but the photographer also must contend with the color imbalances created by masking dyes. A standard color negative image has a pronounced salmon color, attributable to incorporated masking dyes.